The Site of Damage in Amoebae Exposed to X-Rays

The technique of nuclear transfer was used to study the extent to which X-rays damage nuclei and cytoplasms of Amoeba proteus. Results showed that cytoplasm was directly damaged by the radiations. Cytoplasmic damage with doses below 280,000 r was reversible, though damaged cytoplasm could lethally damage normal nuclei. Damage with doses above 300,000 r caused lethal damage and death occurred without division in 0–3 days. Damage to nuclei was probably a combination of a direct action of the radiations and exposure to reversibly damaged cytoplasm. Nuclei were 2·4 times as sensitive as cytoplasms, the LD50 doses being approximately 120,000 r for nuclei and 290,000 r for cytoplasm. Death due to nuclear damage generally occurred without division within 3 to 6 weeks. Nuclear damage cannot be interpreted in terms of the ‘target’ theory.